I just don't get why they would risk their own asses to pull the kid off the fence spike and save him just to hold him captive and kill him later. If their original instinct was to save his life, why decide to kill him now?

They have no choice now.

After spending days torturing and threatening him there is no real chance that he'd join the group. That ship has sailed.

Keep in mind one of the 'compassionate' solutions they discussed was actually enslaving him.

Had they embraced the kid and tried to help him immediately after his friends had abandoned him, they could have been his saviors. They've idiot-decisioned themselves into a corner now. They have to kill him, and it's because their own stupidity put them at risk. The kid could have been beneficial.